New carbon nanotube breakthrough moves them one step closer to mass production

Over the past 15 years, alternative materials like graphene
and carbon nanotubes (CNTs) have been touted as potential solutions to
the silicon scaling problems that have left existing microprocessors
largely stuck between 3.5 – 5GHz. In both cases, research into the new
materials has struggled to create products that could be commercialized.
Neither has advanced to the point where they could be integrated into
large scale manufacturing. Researchers at the University of Wisconsin
have recently announced a breakthrough, however — one that could lead,
in the long term, to profitable solutions that incorporate carbon
nanotubes in shipping products.

One of the critical problems facing carbon nanotubes is the difficulty of putting them precisely where they’re needed. In the past, manufacturers have achieved 88-94% precision. In 2013, we wrote about a new sorting method
that could achieve 95-98% precision — still well below the estimated
99.96% precision the ITRS roadmaps at the time had estimated would be
required for commercial manufacturing. Now, the University of Wisconsin
has claimed it can achieve purity rates of up to 99.98%.

[Constraints] in CNT sorting,
processing, alignment, and contacts give rise to nonidealities when CNTs
are implemented in densely packed parallel arrays such as those needed
for technology… In each scenario, the result has been that, whereas CNTs
are ultimately expected to yield FETs that are more conductive than
conventional semiconductors for logic applications, CNTs, instead, have
underperformed channel materials, such as Si, by sixfold or more.
Likewise, in RF applications, depressed on-state conductance and
imperfect saturation characteristics arising from metallic CNTs and
inter-CNT interactions have limited the maximum frequency of oscillation
and linearity.

The paper goes on to note how even a single
metallic CNT can short-circuit a FET (Field Effect Transistor) and
result in substantially reduced performance. Building arrays of CNTs at
exceptionally high purity isn’t optional — it’s been a fundamental
stumbling block that companies like IBM have sought to solve for years.
In order to reach this milestone, the Wisconsin team uses a technique
it first discussed in 2014 — floating evaporative self-assembly, as
shown below.

Click to enlarge

Here’s how the team describes its findings.

CNT array FETs are demonstrated here with an
on-state conductance of 1.7 mS μm−1 and a conductance per CNT as high as
0.46 G0, which is seven times higher than previous state-of-the-art CNT
array FETs made by other methods. These FETs are nearing the
performance of state-of-the-art single CNT FETs but in the format of an
array in which quasi-ballistic transport is simultaneously driven
through many, tightly packed CNTs in parallel, substantially improving
the absolute current drive of the FETs and, therefore, their utility in
technologies.

The exceptional performance of the arrays
achieved here is attributed to the combined outstanding alignment and
spacing of the CNTs, the postdeposition treatment of the arrays to
remove solvent residues and the insulating side chains of the polymers
that wrap the CNTs, and the exceptional electronic-type purity of the
semiconducting CNTs afforded by the use of polyfluorenes as
CNT-differentiating agents. The performance of previous CNT array FETs
has not been as high, likely because these FETs have not simultaneously
met all of these attributes.

The team believes it has a path forward to continue improving CNT FETs and scaling them up to meet modern semiconductor
manufacturing. The difficulty of this step, however, can’t be
overstated. Right now, the University of Wisconsin is working with
one-inch square wafers. Traditional wafers are between 200-300mm —
vastly larger than the tiny squares of test material that the UW team
worked with. The team also benchmarked its results against 90nm MOSFETs —
and while that’s not a bad choice for a lab test, current semiconductor
manufacturing left 90nm behind more than ten years ago.

If carbon nanotubes could be commercialized,
it could kickstart semiconductor scaling again, at least for certain
applications. But the road between even this breakthrough and mass
commercialization is still a long one — don’t expect to see CNTs
shipping in logic for another 5-10 years, if it ever does. Other niche
applications may find more immediate benefits. But CPUs and SoCs tend to
sit at the very forefront of our technology curve. That makes it
comparatively difficult for new technology to offer large enough
improvements to overtake the industry.

New carbon nanotube breakthrough moves them one step closer to mass production
Reviewed by Chidinma C Amadi
on
9:14 PM
Rating: 5